1. Field of the Invention
This invention relates to the field of integrated circuits and, in particular, to on-chip techniques for asynchronously converting between an asynchronous four-cycle handshaking protocol typically used for on-chip circuits and an asynchronous two-cycle handshaking protocol often used for off-chip signaling.
2. Description of the Prior Art
Conventional very large scale integration--or VLSI--integrated circuit chips, such as MOS and bipolar VLSI chips, often contain hundreds or thousands of on-chip electronic subsystems for performing electronic tasks. The input and output --or I/O--connections between such on-chip circuits and off-chip electronic signals are made via on-chip I/O pads, usually positioned around the periphery of the active circuitry on the chip substrate.
The performance of conventional VLSI chips is often limited by the switching speeds of such I/O pads. One technique for improving performance of these I/O connections is to conserve off-chip I/O bandwidth by converting between transition inefficient handshaking protocols used by on-chip circuits and the transition efficient handshaking protocols used for off-chip signaling.
Handshaking protocols are communication control techniques used to control the transfer of information between circuits. Such protocols typically use a pair of signals, called request and acknowledge signals, to provide a controlled communication path between circuits or subsystems so that the subsystem providing the information initiates the data transfer cycle and only the subsystem designated to receive the information indicates data transfer completion. That is, the circuit wishing to transmit data first sends a request signal and the circuit receiving the data indicates that it has received the data by sending an acknowledge signal.
The two handshaking protocols of primary current concern with respect to MOS and bipolar VLSI chips are the on-chip four-cycle handshaking protocol and the off-chip two-cycle handshaking protocol. The terms "two-cycle" and "four-cycle" refer to the number of logic state or signal transitions that occur between the initial transition of the request signal and the final transition of the acknowledge signal. These signals are said to be asserted, for example, only when they change from a logic zero state to a logic one state and are said to be negated when the transition goes from a logic one to a logic zero state.
The on-chip electronic subsystems, such as logic gate arrays, can be self-timed or asynchronous designs in which the timing of the flow of logic signals from gate to gate is not externally controlled by clock or timing pulses, but rather by the logic signals themselves. Such self-timed circuits often use four-cycle handshaking protocols because of the restorative and return-to-zero properties of this handshaking protocol.
Off-chip electronic systems, such as an interconnection between on-chip electronic subsystems on two separate chips, typically use two-cycle handshaking protocols because twice the data can be transferred using the same number of control signal transitions.